1. Field of the Invention
The present invention is generally directed to an optical device, an optical scanning apparatus and an image forming apparatus, and in particular to an optical device for emitting light, an optical scanning apparatus having the optical device and an image forming apparatus having the optical scanning apparatus.
2. Description of the Related Art
In recent years, vertical cavity surface emitting laser devices, which emit light in a direction perpendicular to the surface of the substrate, have attracted attention because of being less expensive, lower in power consumption, smaller in size and more suitable for two-dimensional devices and having higher performance compared to edge emitting semiconductor laser devices that emit light in a direction parallel to the substrate surface.
The fields of application of the surface emitting laser devices may include light sources for optical writing systems in printers (oscillation wavelength in the 780 nm band), light sources for writing in optical disk apparatuses (oscillation wavelength in the 780 or 850 nm band) and light sources for optical transmission systems, such as a LAN (Local Area Network), using an optical fiber (oscillation wavelength in the 1.3 or 1.5 μm band). Furthermore, the surface emitting laser devices are expected to be applicable as light sources for optical transmission between boards, within a board, between chips in a LSI (Large Scale Integrated circuit) and within a chip in a LSI.
In these fields of application, it is often the case that light emitted from a surface emitting laser device (hereinafter, also referred to as the “emitted light”) is required to have (1) a round cross-sectional shape and (2) a constant polarization direction.
In order to make the cross-sectional shape of the emitted light round, it is necessary to suppress the oscillation of higher-order transverse modes. To do this, various approaches have been made (see, for example, Patent Document 1).
In addition, various approaches to control the polarization direction of the emitted light have been made (see, for example, Patent Document 2).
Furthermore, studies have been made in balancing the suppression of the oscillation of higher-order transverse modes and the control of the polarization direction (see, for example, Patent Documents 3 and 4).
In addition, it is sometimes the case that surface emitting laser devices have a change in the amount of emitted light due to a change in the ambient temperature or degradation of the surface emitting laser devices themselves. Accordingly, it is necessary to monitor the amount of emitted light in order to achieve stable optical scanning and form high-quality images.
As for optical scanning apparatuses using an edge emitting semiconductor laser device, APC (Auto Power Control) is carried out with monitoring the amount of backward emitted light. However, the backward emitted light does not occur in a surface emitting laser device due to its structure. Therefore, in optical scanning apparatuses using a surface emitting laser device, a portion of the beam of light emitted from the light source is received by a detector, such as a photodiode device, as a monitoring beam of light, and APC is carried out based on the reception result (see, for example, Patent Document 5).    [Patent Document 1] Japanese Patent Publication No. 3566902    [Patent Document 2] Japanese Patent Publication No. 3955925    [Patent Document 3] Japanese Laid-open Patent Application Publication No. 2007-201398    [Patent Document 4] Japanese Laid-open Patent Application Publication No. 2004-289033    [Patent Document 5] Japanese Laid-open Patent Application Publication No. H10-051067
However, a power monitoring system disclosed in Patent Document 5 leaves the problem that highly accurate monitoring of the light amount is difficult since the amount of light received by the monitor after being reflected by an optical element is small, which results in a low signal-to-noise ratio (S/N ratio).